Rhodopsin, the photosensitive pigment found in vertebrate rod photoreceptors, consists of a chromophore, 11-cis retinal, covalently joined to a protein, opsin. The overall objective of the project is to provide information on the physiological properties of rhodopsin, particularly those which are linked to the regulation of visual sensitivity within the photoreceptor. We are addressing several problems within this area: (1) The mechanisms by which changes in the content of rhodopsin influence the level of visual sensitivity exhibited by the photoreceptors. Considerable evidence indicates that the dependence of sensitivity on rhodopsin content is far stronger than would be expected from absorptivity changes due to the photolysis (bleaching) of rhodopsin. This relationship between rhodopsin level and sensitivity is expressed during dark adaptation. The intent is to elucidate how the depletion of rhodopsin affects photoreceptor thresholds, both in the dark, and during sustained illumination. A further aim is to examine how artificial visual pigments, formed in previously bleached photoreceptors by treatment of the retina with retinal chromophore, may influence the electrophysiological behavior of the photoreceptor. (2) The utilization of vitamin A compounds (putative precursors of 11-cis retinal) by dark-adapting photoreceptors. In the intact eye, the regeneration of rhodopsin in the receptors relies on the supply of a form of vitamin A by the pigment epithelium; as yet, the identity of this vitamin A derivative is unknown. The objective is to obtain evidence on the nature of this substance, and on its transformations within the vertebrate eye.